Nick Clegg took the extraordinary step of personally urging culture secretary Jeremy Hunt to refer himself to the adviser on the ministerial code, in a bid to avoid further damage to the government during the build up to the vote on the minister’s future.

The deputy prime minister told Hunt that the row over his relationship with BSkyB was hugely damaging and needed to be resolved urgently.

Hunt declined to take the advice, delivered by phone, leaving Clegg unable to support his coalition allies in the Commons vote, and prompting some senior Tories to describe the deputy prime minister as a “traitor”.

Labour’s motion, saying that Hunt had misled the House and breached the ministerial code, was defeated by 290 votes to 252, but the Liberal Democrat abstention caused fresh tensions in the coalition and the row over the beleaguered culture secretary continues to haunt the prime minister.

Speaker John Bercow allowed Labour to accuse Hunt of lying to MPs over his handling of Rupert Murdoch’s takeover bid for BSkyB and Cameron’s critics say the row has raised significant doubts about the prime minister’s judgment.

These concerns will now be magnified by the Observer‘s revelation that Clegg repeatedly tried and failed during increasingly anguished conversations to persuade the prime minister of the need to refer the case to Sir Alex Allan, his adviser.

It is believed that Clegg could not understand the prime minister’s reluctance to refer the case, given his insistence that there was no evidence of the ministerial code being broken by the relationship between Hunt’s office, the News Corp lobbyist Fred Michel, and the Murdochs.

A senior Liberal Democrat source said the deputy prime minister became particularly frustrated after Cameron took the decision to refer Tory chairman Baroness Warsi to Allan for failing to declare a business relationship of a relative who travelled with her on a government trip to Pakistan.

The development pushed Clegg into approaching Hunt, only to be ignored, according to a senior Liberal Democrat source. The source added: “Nick Clegg was frustrated that David Cameron couldn’t see how damaging this was to both himself and the government. He thought that referring Warsi made the position even more untenable than it already was.

“Nick Clegg made his position clear to the prime minister before Hunt’s appearance at the Leveson inquiry. He made it again in the aftermath of Cameron’s decision not to refer and numerous times in the week running up to the vote. He even spoke to Jeremy Hunt personally urging him to refer himself to Sir Alex Allan to get himself out of a corner.”

The source said the Liberal Democrat leadership did not have any problem in believing in Hunt’s innocence but Clegg believed it was wrong to avoid a referral. “No one is making any judgments about culpability,” the source added.

“Nick Clegg values Jeremy Hunt as a collaborative colleague in the coalition. That is not the issue. This was about the Liberal Democrats being unable to support what we felt was the politically unsustainable decision not to refer.

“When it became clear that they wouldn’t help themselves, Nick wasn’t going to expend political capital defending them. We weren’t going to endorse a decision in the House that we weren’t consulted on and didn’t agree with.”

In response to the Liberal Democrat failure to vote against Labour’s motion last week, furious Tory MPs have threatened to derail Clegg’s plans for an elected House of Lords in a revenge attack.

Cameron has played down the Liberal Democrat manoeuvre, telling MPs, “It’s politics”.